David Cameron emerged the main winner of the third debate, and Nick Clegg the main loser, according to a Guardian/ICM poll immediately after the event. The Tory leader led Gordon Brown by six points as the best performer, 35% to 29%. Clegg, winner of the first two debates, followed on 27%.

But Brown held his own on a series of character questions. By a small margin viewers saw him as best potential prime minister, and most likely to be respected around the world. A majority also forgave him for calling a voter in Rochale a bigot. Only 33% said it worsened their view of him; 58% said it made no difference.

Compared with the first debate, Cameron improved his performance by 15 points, and by six points on the second. Clegg's score dropped 24 points on the first and six points on the second. Brown's score rose 10 points on the first debate and was unchanged on the second. While 76% of Conservative supporters thought Cameron won, and 62% of Labour supporters named Brown, only 54% of Lib Dems backed Clegg.

However, the third debate had less effect than the first two. Only 15% of respondents who saw it said it changed the way they planned to vote, while 77% said they had already made up their mind. Viewing figures will emerge today.

The Lib Dems continued to pick up more supporters than any other party, with 39% of the small number of switchers saying they were moving to Clegg's party. But Labour was close behind on 33%, with 15% moving to the Conservatives.

Overall, 36% said Brown would be the best prime minister, against 35% who named Cameron, and 23% Clegg. Brown's lead over Cameron was larger as the leader most likely to make the decisions when the going gets tough, 45% to 32%; Clegg was third on 17%. But the Lib Dem leader remains well ahead as the politician among the three who had changed their party for the better. While 55% named him and 26% Cameron, only 12% said Brown, Even among Labour voters only 31% thought he had changed his party for the better.

Cameron suffers once again as the leader seen as more spin than substance: 47% of people who watched the debate say this about him, the same as after the previous debate. Clegg continues to lead on this measure: only 20% say he is more spin than substance, up one on the previous debate. For Brown the figure is 28%.

Brown is six points ahead of Cameron, 43% to 37%, as the leader most likely to be respected around the world, with Clegg on 15%, although the Tory leader has closed the gap from 10 points at the end of the previous debate. Brown is also ahead by three points, 37% to 34%, as the most decisive leader, with Clegg on 24%.

Brown's gaffe seems to have gone down worst with people already likely to vote Conservative: 55% of Tory supporters said it worsened their view of him, against 33% of all voters, and only 14% of Labour definites.

While the consensus among the instant polls gave the event to Cameron, most others put Clegg ahead of Brown. Populus for the Times put Clegg in joint first place with Cameron on 38%, with Brown on 25%. YouGov for the Sun awarded Cameron the biggest winning margin, of nine points. It had the Tory leader on 41%, Clegg on 32%, and Brown on 25%. An ITV News/ComRes poll had Cameron ahead on 35%, Clegg on 33%, and Brown on 26%. Clegg had won the previous two debates in the corresponding poll.

ComRes also polled on the effect of Brown's remarks in Rochdale. Some 7% of those questioned said they had planned to vote Labour and now would not; another 8% were planning to vote Labour but now are undecided.

Polling company Angus Reid said that Cameron won on 36%, with Clegg on 30% and Brown on 23%. But its analysis also said that Clegg "appears to have done a better job connecting with undecided voters". Some 37% of undecided voters believe Clegg won the debate, according to Angus Reid, with 25% believing Cameron won it, and 22% Brown. A third of those undecided voters say they are now more likely to vote for the Liberal Democrats. Clegg was ahead of his rivals in being seen as having the common touch and understanding the needs of people.

ICM interviewed a sample 510 adults aged 18+ by phone immediately after the final debate finished. Respondents had previously been selected at random and had agreed to be contacted. The data is weighted to the profile of people who said they would watch the debate.

Editorial: If the Guardian had a vote it would be cast enthusiastically for the Liberal Democrats. But under our discredited electoral system some people may – hopefully for the last time – be forced to vote tactically